Since the mesencephalic reticular formation (MRF) is a site where morphine acts to produce analgesia, it is a site where the mechanism of action of morphine can be studied. The neurotransmitter(s) involved in the nociceptive pathway in the MRF will be tentatively identified by injecting neurotoxic agents and selective antagonists intercerebrally (IC) into the MRF. When one of these agents alters the latency or threshold for the behavioral endpoint for nociception or the analgesic effect or morphine, the transmitter affected by this agent will be assumed to be a putative nociceptive transmitter. This transmitter will then be administered microiontophoretically to determine if it will mimic the acceleration in neuronal firing produced by nociceptive stimulus. Furthermore, morphine will be ejected microiontophoretically to determine if it will block both the acceleration in neuronal firing produced by the transmitter and that produced by the nociceptive stimulus. Areas which project to sites in the MRF where morphine acts to produce analgesia will be identified by using retrograde tracing with horseradish peroxidase. The ability of morphine to block changes in neuronal firing elicited by a nociceptive stimulus in these areas will be studied as well as the mechanism by which it does so. These areas will be stimulated electrically to determine the effects on neuronal firing in the MRF. Neuronal firing will also be recorded in freely moving rats which are on a electronic hotplate to determine if an increase in neuronal firing in the MRF (as well as other areas in the nociceptive pathway) can be related to behavioral endpoints associated with nociception (e.g., paw lick). Finally, the mechanism of action by which morphine produces tolerance and dependence at a cellular level will be explored using both acute microiontophoretic experiments and single unit recording in the freely moving, morphine tolerant-dependent rats that are undergoing spontaneous and naloxone-induced withdrawal.